1. Field
This invention is concerned generally with the preparation of serum standards useful for radioassays and specifically with the preparation of a serum standard useful for the radioimmunoassay of T.sub.4.
2. Prior Art
Radioassays of T.sub.4 in human serum commonly require a human serum based standard substantially free of T.sub.4. Although free T.sub.4 can be removed from human serum by known means (e.g., repetitive exposure to anion exchange resins), it is known that such serum commonly also contains TBG which has a relatively high binding affinity for T.sub.4. When T.sub.4 is bound to TBG, it is difficult to extract by conventional means since it is not free. The removal or inactivation of TBG facilitates T.sub.4 removal but conventional methods have various known disadvantages. For example, various blocking agents can be used to prevent T.sub.4 from binding to TBG, thereby facilitating T.sub.4 extraction or precluding TBG interference in T.sub.4 assays. Other methods of preparing a serum standard substantially free of T.sub.4 include the use of a charcoal absorbant which is relatively messy and time consuming or the use of anion exchange resin which, though relatively cleaner, requires a minimum of four days contact with the serum. Although it is known that certain crystalline inorganics can be used to preferentially adsorb T.sub.4 from serum (e.g. U.S. Pat. Nos. 3,666,854; 3,743,482; and 3,775,615 to Eisentraut) and that thyroxine can be extracted with an acidic ion exchange resin (e.g. Ingbar et al., Endocrinology, V. 61, pp. 321-326, 1957), we have found that by combining those general techniques under limited conditions, substantially all T.sub.4 and TBG can be removed from serum in a quick, simple, two step reaction which, surprisingly, takes less than eight hours.